The invention relates to a modular multilevel converter and a startup control method thereof. The method includes the following steps: carrying out pre-charging; unlocking the converter; setting a voltage balancing control link, taking the output of the voltage balancing control link as the command value of active current in a current control link to get the modulation voltage of each bridge arm, dividing the sub module voltage command value of the voltage balancing control link by the modulation voltage of each bridge arm to get the number of sub modules needing to be switched on for each bridge arm, and gradually increasing the sub module voltage command value of the voltage balancing control link to further charge the sub modules to a rated value; and finally, carrying out active control mode conversion, and normally unlocking and operating the converter. At the beginning of converter unlocking, there are a large number of sub modules participating in modulation for each bridge arm, so that a valve AC/DC port voltage closer to the actual AC/DC system voltage is formed when the sub module voltage is low, and the voltage and current impact in the startup process is reduced.